Extrusive rocks of Miocene and younger age have been dredged from the submerged insular slopes of the arcuate, 2,220-km-long Aleutian Ridge. Hornblende dacite porphyry recovered at station 70-B29 (lat 52.6°N, long 174.8°E; depth, 700 m) was extruded less than 610,000 yr ago. The dacite crops out approximately 80 km west of Buldir Island, the westernmost volcanic edifice of the 2,550-km-long Aleutian volcanic chain. The submerged dacite extends the westward limit of this chain of eruptive centers, which are the product of a distinct phase of late Cenozoic (chiefly early Pliocene to present) volcanism. This part of the ridge is not associated with a north-dipping Benioff zone, a fact that may imply that arc-type calc-alkalic magma can be emplaced along sectors of the ridge either obliquely underthrust by the Pacific plate or in strike-slip contact with it (western 800 km).

Vesicular augite andesite from dredge site 70-B49 (lat 55.7°N, long 165.2°E; depth, 1,600 m) at the western end of the Aleutian Ridge is greater than 8.8 m.y. old. It was probably extruded in middle Miocene time. At this same time (10 to 16 m.y. ago) a distinct phase of calc-alkalic plutonism and subaerial volcanism affected the ridge's eastern or Aleutian Island sector (long 164°W to 172°E). The age of the 70-B49 andesite and our interpretation of the magmatic history of the nearby Komandorsky Islands imply that this episode of volcanism extended along the full length of the ridge and may have been the last major pulse of igneous activity to do so. The middle Miocene ridge, unlike the present one, was presumably everywhere underthrust by Pacific plate lithosphere.

The Pliocene and Pleistocene episode of Aleutian activity is coeval with volcanism that affected much of the North Pacific perimeter. A similarly widespread pulse of igneous activity and tectonism in middle Miocene time is suggested by other dated rocks and regional geologic mapping. Determination of the timing of major pulses of North Pacific activity bears on the important question of episodic magma generation during what is presumed to be continuous plate subduction.

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